Use string.ascii_letters instead of string.letters (SF bug #226706).
diff --git a/Tools/idle/AutoExpand.py b/Tools/idle/AutoExpand.py
index 09f34b3..40d39f3 100644
--- a/Tools/idle/AutoExpand.py
+++ b/Tools/idle/AutoExpand.py
@@ -21,7 +21,7 @@
          ]),
     ]
 
-    wordchars = string.letters + string.digits + "_"
+    wordchars = string.ascii_letters + string.digits + "_"
 
     def __init__(self, editwin):
         self.text = editwin.text